As anti-reflection films for visible light, multi-layered films wherein plural transparent layers made of metal compounds (e.g., metal oxides) are superposed one on another have been employed. In the case that a monolayer film is employed instead of the multi-layered film, the monolayer film cannot effectively lower reflection of light having wide wavelength region (e.g., visible light), though the monolayer film can lower reflection of monochromatic light.
In the multi-layered film, increase of the number of the layers can enlarge a wavelength region of light to which the multi-layered film can apply. Therefore, the known anti-reflection film is the multi-layered film composed of two or more metal oxide layers which are deposited by means of deposition. However, formation of the multi-layered film requires a complicated process comprising a number of physical or chemical deposition procedures which correspond to the number of the metal compound layers having predetermined refractive index and thickness, under highly precise control of the thickness for each layer. Further, on the multi-layered film, a fluororesin layer is needed to be formed to improve stain resistance (e.g., resistance to formation of fingerprint onto the surface of the film).
Alternatively to the multi-layered film, it is known to use, as the anti-reflection film, a film in which refractive indices to air gradually vary in the thickness direction. Japanese Patent Provisional Publication No. 2(1990)-245702 describes an anti-reflection film comprising MgF.sub.2 micro particles and SiO.sub.2 micro particles having refractive index between those of glass and MgF.sub.2, in which blending ratio of the MgF.sub.2 micro particles and SiO.sub.2 micro particles varies in the thickness direction. In more detail, the anti-reflection film is prepared by coating a liquid containing a mixture of the MgF.sub.2 micro particles and SiO.sub.2 micro particles on a glass plate so as to reduce a ratio of the SiO.sub.2 micro particles to MgF.sub.2 micro particles. Thus formed anti-reflection film shows a little variation of refractive indices between its bottom and the upper surface of the glass plate. Therefore, the anti-reflection film shows a high anti-reflection effect.
Japanese Patent Provisional Publication No. 5(1993)-13021 also describes an anti-reflection film comprising two layers of MgF.sub.2 micro particles and SiO.sub.2 micro particles dispersed in ethyl silicate, a first layer containing MgF.sub.2 micro particles and SiO.sub.2 micro particles at ratio of 7/3 and a second layer containing MgF.sub.2 micro particles and SiO.sub.2 micro particles at ratio of 1/1. The first layer has a refractive index of 1.42 and the second layer has a refractive index of 1.44. Thus, the anti-reflection film does not show a satisfactory anti-reflection effect.
Japanese Patent Provisional Publication No. 7(1995)-92305 describes an anti-reflection film which comprises polymer particles (e.g., refractive index: 1.428) composed of a core and shell therearound and which has a structure composed of an uneven surface portion of a low refractive index comprising air and the polymer particles and a portion comprising only the polymer particles. The core is composed of copolymer of methyl methacrylate, methacrylic acid, trifluoroethyl acrylate and N-isobutoxymethyl acrylate, and the shell is composed of copolymer of styrene, acrylic acid and butyl acrylate.
Japanese Patent Provisional Publication No. 7(1995)-168006 describes an anti-reflection film which has a structure composed of an uneven surface portion of a low refractive index comprising air and particles (e.g., MgF.sub.2), a portion (intermediate refractive index) comprising only the particles thereunder, and a portion comprising the particles and binder.
Japanese Patent Provisional Publications No. 2(1990)-245702, No. 5(1993)-13021, No. 7(1995)-92305 and No. 7(1995)-168006 mentioned above, all describe a film in which the refractive indices gradually vary in the thickness direction. However, the preparation of these films require complicated procedures and skilled art, and further the films do not show a satisfactory anti-reflection effect.